Aqueous coating composition and coating

ABSTRACT

Desclosed is an aqueous coating composition characterized in that the composition consists essentially of: 
     (A) a resin for use in aqueous coating compositions, 
     (B) a urethane resin emulsion, and 
     (C) crosslinking agent, 
     the component (B) being an aqueous dispersion of a polyurethane resin prepared by reacting (a) an aliphatic polyisocyanate and/or an alicyclic polyisocyanate, (b) a high-molecular-weight polyol, (c) an α,α-dimethylolmonocarboxylic acid, and when required, (d) a chain lengthening agent and/or a polymerization terminator, and subsequently neutralizing the carboxyl with (e) a primary monoamine and/or a secondary monoamine.

The present invention relates to a novel aqueous coating composition anda coating method with use of the composition.

It is already known to form overcoats by the two-coat one-bake methodusing an aqueous base coating composition and a clear coatingcomposition of the organic solvent type. The aqueous base coatingcomposition to be used is a composition consisting essentially of anacrylic resin emulsion, a urethane resin emulsion neutralized with atertiary amine and a crosslinking agent (Unexamined Japanese PatentPublication HEI 1-287183).

The aqueous coating composition, when used, forms overcoats which areexcellent in distinctness-of-image gloss, smoothness, weatherresistance, etc. and which have improved resistance to chipping.However, if the clear coating composition used is a thermosettingcomposition of high solids content containing an acid catalyst, theclear coating formed is not curable sufficiently. This results in thedrawback that the overcoat is low in resistance to weather, solvents andwater and in interlayer adhesion and exhibits an impaired appearance onfinishing with respect to distinctness-of-image gloss and smoothness.

When examined, these faults are found attributable to the phenomenonthat the tertiary amine which is present in the aqueous base coatingcomposition to serve as the neutralizing agent bleeds into the clearcoating. Accordingly, it may be attempted to dry the coating of theaqueous composition by heating before the application of the clearcomposition to vaporize the tertiary amine, whereas the amine, whichforms a salt with the carboxyl of the resin, remains in the coating.Thus, the problem remains unsolved.

The thermosetting composition of high solids content used as the clearcoating composition in the two-coat one-bake method, as well as theaqueous base coating composition, serves to control air pollution and toensure savings in resources. It is therefore important for industries toobtain aqueous coating compositions which are satisfactorily usable incombination with the thermosetting coating composition of high solidscontent.

An object of the present invention is to provide an aqueous base coatingcomposition which is satisfactorily usable along with thermosettingcoating compositions of high solids content.

Another object of the present invention is to provide a coating methodwith use of the aqueous base coating composition.

Other features of the present invention will become apparent from thefollowing description.

The present invention provides an aqueous coating composition whichconsists essentially of:

(A) a resin for use in aqueous coating compositions,

(B) a urethane resin emulsion, and

(c) crosslinking agent,

the component (B) being an aqueous dispersion of a polyurethane resinprepared by reacting (a) an aliphatic polyisocyanate and/or an alicyclicpolyisocyanate, (b) a high-molecular-weight polyol, (c) anα,α-dimethylolmonocarboxylic acid, and when required, (d) a chainlengthening agent and/or a polymerization terminator, and subsequentlyneutralizing the carboxyl with (e) a primary monoamine and/or asecondary monoamine.

The present invention also provides a two-coat one-bake coating methodcomprising coating a substrate with a base coating compositioncomprising a coloring pigment and/or a metallic pigment, applying aclear coating composition to the resulting coating without curing thecoating to form a transparent coating thereon, and thereafter heatingthe two coatings for curing, the coating method being characterized inthat the above aqueous coating composition is used as the base coatingcomposition.

First, the aqueous coating composition of the invention will bedescribed.

Component (A): Resin For Use In Aqueous Coating Compositions

This resin is the basic component of the cured coating to be obtained.Examples of useful resins are acrylic resins, alkyd resins(polyester-containing resins, the same as hereinafter), epoxy resins,fluorine-containing resins and the like, as rendered soluble in water ordispersible in water. Preferably such resins are those having hydroxylor carboxyl in the molecular structure. Among the examples given,acrylic resins are preferred. Acrylic resins a) to d) are describedbelow in detail for use in the present invention.

a) Water-Soluble Acrylic Resins

Useful examples are neutralized products of acrylic resins which areprepared by copolymerizing a carboxyl-containing vinyl monomer (M-1), ahydroxyl-containing vinyl monomer (M-2) and other vinyl monomer (M-3),and having an acid value of about 20 to about 150, a hydroxyl value ofabout 20 to about 200 and a number average molecular weight of about3,000 to about 100,000.

The carboxyl-containing vinyl monomer (M-1) is a compound having atleast one carboxyl group and one polymerizable unsaturated bond in themolecule. Examples of such monomers are acrylic acid, methacrylic acid,crotonic acid, maleic acid, itaconic acid and the like.

The hydroxyl-containing vinyl monomer (M-2) is a compound having onehydroxyl group and one polymerizable unsaturated bond in the molecule.The hydroxyl group primarily acts as a functional group reactive withcrosslinking agents. Preferably, the monomer is a monoester of acrylicacid or methacrylic acid with a dihydric alcohol having 2 to 10 carbonatoms. Examples of such monoesters are 2-hydroxyethyl acrylate,2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropylmethacrylate and the like.

Other vinyl monomer (M-3) is a compound other than the monomers (M-1)and (M-2) and having one polymerizable unsaturated bond in the molecule.Compounds (1) to (8) are given below as examples of such compounds.

(1) Monoesters of acrylic acid or methacrylic acid with a monohydricalcohol having 1 to 20 carbon atoms, such as methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate,propyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, cyclohexyl acrylate, cyclohexylmethacrylate, lauryl acrylate, lauryl methacrylate, etc.

(2) Aromatic vinyl monomers such as styrene, α-methylstyrene,vinyltoluene, etc.

(3) Glycidyl-containing vinyl monomers which are compounds having oneglycidyl group and one polymerizable unsaturated bond in the molecule,such as glycidyl acrylate, glycidyl methacrylate and the like.

(4) Nitrogen-containing alkyl (having 1 to 20 carbon atoms) acrylatessuch as dimethylaminoethyl acrylate and dimethylaminoethyl methacrylate.

(5) Amide compounds having a polymerizable unsaturated bond, such asacrylic amide, methacrylic amide, dimethylacrylic amide,N,N-dimethylpropylacrylic amide, N-butoxymethylacrylic amide,N-methylolacrylic amide, N-methylolmethacrylic amide anddiacetoneacrylic amide.

(6) Vinyl compounds such as vinyl acetate, vinyl propionate and vinylchloride.

(7) Nitrile compounds containing a polymerizable unsaturated bond suchas acrylonitrile and methacrylonitrile.

(8) Diene compounds such as butadiene and isoprene.

One or at least two of these other vinyl monomers (M-3) can be used.

The vinyl monomers (M-1) to (M-3) can be copolymerized by a knownmethod. If less than about 20 in acid value, the acrylic resin obtainedis difficult to dissolve in water, whereas if the acid value is overabout 150, it is likely that the coating formed will have impairedproperties due to the influence of the residual carboxyl.

It is desired that the acrylic resin thus obtained be neutralized with aprimary monoamine and/or a secondary monoamine examples of such arementioned below, and thereby made soluble in water.

b) Water-Dispersible Acrylic Resin-1

This resin is an acrylic resin in the form of fine particles of 0.05 to1.0μ in mean size and prepared by subjecting a vinyl monomer to emulsionpolymerization in the presence of a surfactant or like dispersionstabilizer. The resin is used as dispersed in water.

The vinyl monomer to be subjected to emulsion polymerization ispreferably one selected from among the monomer (M-1), monomer (M-2) andmonomer (M-3) mentioned above. Further when required, a small amount ofpolyvinyl monomer (M-4) having at least two polymerizable unsaturatedbonds in the molecule is used conjointly. A water-dispersible acrylicresin is then obtained which is crosslinked within particles. This resinis desirable since the coating obtained has improved properties.

Examples of useful polyvinyl compounds (M-4) are ethylene glycoldiacrylate, ethylene glycol dimethacrylate, triethylene glycoldiacrylate, tetraethylene glycol dimethacrylate, 1,6-hexanedioldiacrylate, divinylbenzene, trimethylolpropane triacrylate and the like.It is desired that the two or more unsaturated double bonds present inthese compounds be not greatly different in reactivity. Theabove-mentioned diene compound (8) is not used when the compound (M-4)is used.

Preferably, the water-dispersible acrylic resin thus prepared is alsoneutralized with the primary and/or second monoamine(s) to be mentionedbelow.

c) Water-Dispersible Acrylic Resin-2

This resin is an acrylic resin in the form of fine particles dispersedin water and stabilized with a stabilizer polymer. The resin, i.e., theaqueous dispersion, is an emulsion of the core/shell type wherein theparticles are cores, and the stabilizer polymer forms shells.

More specifically, this emulsion is prepared by subjecting a vinylmonomer completely or almost completely free from the carboxy-containingvinyl monomer (M-1) to emulsion polymerization first, thereafter addinga vinyl monomer containing the vinyl monomer (M-1) to the resultingpolymer, and subjecting the mixture to emulsion polymerization. Whenneutralized with the primary and/or secondary monoamine(s) to bementioned later, the emulsion is given an increased viscosity and istherefore desirable in view of coating efficiency.

d) Water-Dispersible Acrylic Resin-3

This resin is in the form of an emulsion of the core/shell typecomprising crosslinked polymer particles (cores) and a polymer (shells)for stabilizing the particles, the core portion being chemically bondedto the shell portion.

The core portion is bonded to the shell portion preferably for causingthe surface of the core portion to retain a hydrolyzable functionalgroup or silanol group, then introducing a polymerizable unsaturatedbond into the group, copolymerizing a vinyl monomer containing thecarboxyl-containing vinyl monomer (M-1) with the unsaturated bond (toform the shell portion) and thereafter neutralizing the carboxyl groupsof the shell portion.

Being thixotropic, the emulsion has the features of not permitting thecoating to sag even under a high humidity, exibiting no change whenincorporating an organic solvent added thereto and forming coatingswhich are excellent in smoothness, gloss, water resistance and adhesion.

This emulsion of the core/shell type is prepared by the following steps(I) to (III).

(I) A silane monomer (M-5) having hydrolyzable functional groups and/orsilanol groups, and a polymerizable unsaturated bond, ahydroxyl-containing vinyl monomer (M-2) and a vinyl monomer (M-6) otherthan these monomers are reacted in an aqueous medium to prepare anemulsion comprising a three-dimensionally crosslinked polymer in theform of fine particles and dispersed in water. The fine particles formthe core.

(II) Another portion of the silane monomer (M-5) and/or allyl(meth)acrylate (M-7) are/is reacted with the particulate polymer in theemulsion.

Presumably in the step (II), the silane monomer (M-5) undergoes acondensation reaction with the functional group on the surface of theparticulate polymer, and the allyl (meth)acrylate (M-7) copolymerizeswith the unreacted polymerizable unsaturated bond remaining in theparticulate polymer. The polymerizable unsaturated bond can beintroduced into the surface of the particulate polymer by either ofthese reactions.

(III) A vinyl monomer (M-8) containing a carboxyl-containing vinylmonomer (M-1) is copolymerized with the polymer within the emulsionresulting from the reaction of the step (II), followed by theneutralization of the carboxyl. The neutralized copolymer serves as thepolymer for stabilizing the dispersion of the particulate polymer, andcorresponds to the shell portion. In the step (III), the vinyl monomer(M-8) copolymerizes with the polymerizable unsaturated bonds derivedfrom the silane monomer (M-5) and/or the allyl (meth)acrylate (M-7) andpresent on the surface of the particulate polymer resulting from thereaction of the step (II).

The silane monomer (M-5) for use in the step (I) is preferably acompound wherein three hydrolyzable functional groups and/or silanolgroups, and a residue group having one polymerizable unsaturated bondare attached to Si in the molecule.

This compound is represented by the formula (R₁)₃ -Si-X (wherein R₁ is ahydrolyzable functional group and/or silanol group, and X is a residuegroup having polymerizable unsaturated bond). The silane monomer (M-5)primarily has the function of forming the core portion through internalcrosslinking.

In the above formula, examples of hydrolyzable functional groupsrepresented by R₁ are alkoxyl having 1 to 12 carbon atoms, alkoxyalkoxylhaving 3 to 15 carbon atoms, and alkanoyloxy having 1 to 12 carbon atomsand examples of groups X are CH₂ =CH-- and ##STR1## Exemplary of R₂ areH and CH₃ --, and n is an integer of 2 to 10.

More specific examples of silane monomers (M-5) arevinyltrimethoxysilane, vinyltriethoxysilane,acryloxyethyltrimethoxysilane, methacryloxyethyltrimethoxysilane,acryloxypropyltrimethoxysilane, methacryloxypropyltrimethoxysilane,acryloxypropyltriethoxysilane,methacryloxy-propyltriethoxysilane,vinyltris(β-methoxyethoxy)silaneand the like. Among these, especially preferable arevinyltrimethoxysilane,γ-acryloxypropyltrimethoxysilane andγ-methacryloxypropyltrimethoxysilane.

The hydroxyl-containing vinyl monomer (M-2) to be used in the step (I)is the one already mentioned. Exemplary of the "vinyl monomer (M-6)other than these monomers" are the aforementioned monomers (M-1) to(M-5) other than the monomers (M-5) and (M-2).

The step (I), i.e., the copolymerization of the silane monomer (M-5),hydroxyl-containing monomer (M-2) and other monomer (M-6) in an aqueousmedium to obtain the three-dimensionally crosslinked particulatepolymer, can be conducted by the following emulsion polymerizationprocesses (i) to (iii) given below which are already known.

(i) A mixture of the monomers is slowly added dropwise to an aqueousmedium, prepared by adding a surfactant to water, with stirring in aninert gas atmosphere to effect copolymerization at a predeterminedtemperature.

(ii) A mixture of the monomers is emulsified in an aqueous medium first,and the emulsion is slowly added dropwise to water to effectcopolymerization at a predetermined temperature.

(iii) A small amount of one of the monomers or of a mixture thereof issubjected to seed polymerization first, and the polymer and theremaining portions of the monomers are then subjected to emulsionpolymerization by the process (i) or (ii).

Among these processes, the process (iii) is preferable since the polymerthen obtained is smaller in particle size and imparts improvedresistance to sagging and smoothness to the coating obtained.

These emulsion polymerization processes are conducted preferably in thepresence of a radical polymerization initiator.

The proportions of the monomers for use in the steps (I) and (III) willnow be described. The preferred proportions of the combined amount ofall the monomers in the step (I) and the amount of the monomer in thestep (III) are 30 to 95 wt. %, especially 60 to the step 90 wt. %, ofthe monomers of the step (I), and 70 to 5 wt. %, especially 40 to 10 wt.%, of the monomer of the step (III), based on the total of theseamounts.

Further all the monomers used in the step (I) are the silane monomer(M-5), the hydroxyl-containing vinyl monomer (M-2) and the other vinylmonomer (M-6). Based on the combined amount of these monomers, it ispreferable to use 0.5 to 20 wt. %, more preferably 1 to 10 wt. %, of thesilane monomer (M-5), 1 to 30 wt. %, more preferably 2 to 20 wt. %, ofthe hydroxyl-containing vinyl monomer (M-2) and 98.5 to 50 wt. %, morepreferably 97 to 70 wt. %, of the other vinyl monomer (M-6).

Further the monomer for use in the step (III) is the vinyl monomer (M-8)containing the carboxyl-containing vinyl monomer (M-1). Preferably, themonomer (M-8) contains 1 to 50 wt. %, more preferably 3 to 30 wt. %, ofthe monomer (M-1). The monomer (M-8) comprises the monomer (M-1), thehydroxyl-containing vinyl monomer (M-2) and at least one of the monomers(1) to (8) exemplified for the monomer (M-3). Preferably, the monomer(M-2) contains up to 30 wt. %, more preferably up to 25 wt. %, of themonomer (M-8).

The silane monomer (M-5) and polyvinyl compound (M-4) can be used incombination with the monomer (M-8), in an amount of up to 10 wt. % basedon the monomer (M-8).

It is considered that the particulate polymer of the emulsion preparedby the step (I) is threedimensionally crosslinked primarily owing to acarbon-carbon bond due to the polymerizable unsaturated bond and--Si--O--Si-- bond due to the silane monomer (M-5). It appears that thesurface of the particulate polymer has attached thereto unreactedhydrolyzable functional groups and/or silanol groups due to the silanemonomer (M-5). Furthermore, hydroxyl groups due to the monomer (M-2) arealso present on the surface. The particulate polymer obtained by thestep (I) is preferably 10 to 500 nm, more preferably 30 to 300 nm, inparticle size although the size differs depending on the kind and amountof surfactant used and the method of polymerization.

The step (II) serves to introduce the polymerizable unsaturated doublebond into the surface of the particulate polymer. The polymer having thepolymerizable unsaturated bond introduced therein and obtained by thestep (II) will hereinafter be referred to briefly as "unsaturatedparticulate polymer."

Although the ratio of the silane monomer (M-5) to the particulatepolymer in the step (II) is not limited specifically, it is desirable touse 0.5 to 2 moles of the silane monomer (M-5) in the step (II) per moleof the silane monomer (M-5) used in the step (I) (usually 50 to 200parts by weight of the latter per 100 parts by weight of the former).

It is desirable to use the monomer (M-5) and the monomer (M-7) incombination, and these monomers can each be used in the same ratio asstated above.

In the step (III), the unsaturated particulate polymer obtained by thestep (II) is copolymerized with the vinyl monomer (M-8) containing thecarboxyl-containing vinyl monomer (M-1), and the carboxyl is thenneutralized to form the shell portion.

In this step, the copolymer (corresponding to the shell portion) whichis formed by the vinyl monomer (M-8) and which is predominantly linearis attached to the unsaturated particulate polymer (corresponding to thecore portion) resulting from the step (II).

The step (III) also includes a procedure for copolymerizing the vinylmonomer (M-8) with the unsaturated particulate polymer to form a shellportion and further neutralizing the carboxyl in the shell portion. Itis desirable to use the primary or secondary amine to be stated later asthe neutralizing agent.

According to the present invention, it is most suitable to use thewater-dispersible acrylic resin-3.

Component (B): Urethane Resin Emulsion

A first component (a) for forming the component (B) comprises analiphatic polyisocyanate and/or an aliphatic polyisocyanate. Thealiphatic or alicyclic polyisocyanate is a compound having at least twoisocyanate groups, and an aliphatic carbon group or alicyclic carbongroup in the molecule. Examples of such compounds are aliphaticdiisocyanates having 2 to 12 carbon atoms, such as hexamethylenediisocyanate (HDI), 2,2,4-trimethyl-hexane diisocyanate and lisinediisocyanate; alicyclic diisocyanates having 4 to 18 carbon atoms, suchas 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI),4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI),methylcyclohexane diisocyanate,isopropylidenedicyclohexyl-4,4'-diisocyanate and1,3-diisocyanatomethylcyclohexane (hydrogenated XDI); aliphaticdiisocyanates having an aromatic ring such as xylylene diisocyanate(XDI) and tetramethylxylylene diisocyanate (TMXDI); modified products ofthese diisocyanates (as modified with carbodiimide, uretidione,uretonimine, and biuret and/or isocyanurate); and mixtures of at leasttwo of these examples. Preferable among these are HDI, IPDI,hydrogenated MDI and TMXDI.

Use of an aromatic polyisocyanate as the component (a) is not desirablesince the coating is then prone to yellow, and is also liable todiscolor by being affected by ultraviolet rays.

Examples of useful high-molecular-weight polyols (b) are polyetherpolyols such as those obtained by subjecting an alkylene oxide (ethyleneoxide, propylene oxide, butylene oxide or the like) and/or aheterocyclic ether (tetrahydrofuran or the like) to polymerization orcopolymerization, and including, for example, polyethylene glycol,polypropylene glycol, polyethylene-polypropylene (block or random)glycol, polyethylene-tetramethylene glycol (block or random),polytetramethylene ether glycol and polyhexamethylene ether glycol;polyester polyols such as those obtained by subjecting an aliphaticdicarboxylic acid (succinic acid, adipic acid, sebacic acid, glutaricacid, azelaic acid or the like) and/or an aromatic dicarboxylic acid(isophthalic acid, terephthalic acid or the like), and alow-molecular-weight glycol (ethylene glycol, propylene glycol,1,4-butanediol, 1,6-hexanediol, 3-methyl-1,5- pentanediol, neopentylglycol, 1,4-dihydroxymethylcyclohexane or the like) to condensationpolymerization, and including, for example, polyethylene adipate diol,polybutylene adipate diol, polyhexamethylene adipate diol, polyneopentyladipate diol, polyethylene/butylene adipate diol, poly-3- methylpentaneadipate diol and polybutylene isophthalate diol; polylactone polyolssuch as polycaprolactone diol or triol and poly-3-methylvalerolactonediol; polycarbonate diols such as polyhexamethylene carbonate diol;polyolefin polyols such as polybutadiene glycol or hydride thereof; andmixtures of at least two of such polyols. Among thesehigh-molecular-weight polyols, preferable are polyester polyols,polylactone polyols, polycarbonate polyols and mixtures of such polyols,or mixtures of such a polyol and a polyether polyol. Thehigh-molecular-weight polyol has a molecular weight usually of 500 to5000, preferably 1000 to 3000.

The α,α-dimethylolmonocarboxylic acid (c) is a component used forintroducing anionic hydrophilic groups into the composition to dispersethe polyurethane resin in water with good stability. Examples of suchacids are α,α-dimethylolacetic acid, α,α-dimethylolpropionic acid,α,α-dimethylolbutyric acid and the like, among whichα,α-dimethylolpropionic acid is desirable. The amount of theα,α-dimethylolmonocarboxylic acid is preferably 0.3 to 5 wt. %, morepreferably 0.5 to 3 wt. %, calculated as carboxyl groups (--COOH), basedon the urethane resin which is prepared by reacting the components (a)to (c). If the amount is less than 0.3 wt. %, it is difficult to obtaina stable emulsion, whereas amounts exceeding 5 wt. % render the polymerhighly hydrophilic to give the emulsion a high viscosity, furtherentailing the likelihood that the coating obtained will have lower waterresistance.

In preparing the component (B), the chain lengthening agent and/orpolymerization terminator (d) may be used when required. Examples ofuseful chain lengthening agents are low-molecular-weight polyols andpolyamines. Examples of useful low-molecular-weight polyols are glycolsexemplified as materials for the aforementioned polyester polyols andalkeylene oxide low-mole adducts thereof (less than 500 in molecularweight); alkylene oxide low-mole adducts (less than 500 in molecularweight) of bisphenol; trihydric alcohols such as glycerin,trimethylolethane, trimethylolpropane and alkylene oxide low-moleadducts (less than 500 in molecular weight) of such alcohols; andmixtures of at least two of these polyols. Examples of useful polyaminesare aliphatic polyamines such as ethylenediamine,N-hydroxyethylethylenediamine, tetramethylenediamine,hexamethylenediamine and diethylenetriamine; alicyclic polyamines suchas 4,4'-diaminodicyclohexylmethane, 1,4-diaminocyclohexaneandisophoronediamine; aliphatic polyamines having an aromatic ring, such asxylylenediamine and tetramethylxylylenediamine; aromatic polyamines suchas 4,4'-diaminodiphenylmethane, tolylenediamine, benzidine andphenylenediamine; and mixtures of at least two of these polyamines. Theamount of the chain lengthening agent is usually 0.3 to 30 wt. %,preferably 0.5 to 20 wt. %, based on the high-molecular-weight polyol(b).

Examples of polymerization terminators are low-molecular weightmonohydric alcohols (methanol, butanol, cyclohexanol and the like),monovalent alkylamines (mono- and di-ethylamines, mono- anddi-butylamines and the like), alkanolamines (mono- and di-ethanolaminesand the like), etc.

Examples of primary and secondary monoamines (e) for use in neutralizingcarboxyl groups are ammnonia; lower alkylamines such as methylamine,ethylamine, isopropylamine, n-butylamine, dimethylamine, diethylamine,diisopropylamine and di-n-butylamine; alicyclic amines such ascyclohexylamine; hetetrocyclic amines such as morpholine and pyridine;alkanolamines such as monoethanolamine, diethanol amine,monoisopropanolamine, diisopropanolamine, methylethanolamine andmethylisopropanolamine; and mixtures of at least two of such amines.Among these, preferable are secondary alkanolamines. The monoamine isused usually in an amount of 0.5 to 1.5 equivalents, preferably 0.7 to1.3 equivalents, per equivalent of carboxyl.

The equivalent ratio of the component (a) to the components (b), (c) and(d) for use in the reaction for forming the polyurethane is usually 0.7to 1.3, preferably 0.8 to 1.2.

The urethane resin emulsion (B) is prepared by reacting the components(a), (b) and (c), along with the component (d) when required, in thepresence or absence of an organic solvent by the one-shot method ormultistage method to form a carboxyl-containing polyurethane resin,admixing the resin with water after neutralization or while beingneutralized with the monoamine (e) to obtain a dispersion, anddistilling off the solvent when so required. The emulsion is an aqueousdispersion of self-emulsifiable polyurethane resin having a meanparticle size of about 0.001 to about 1.0μ. The polyurethane formingreaction is conducted usually at a temperature of 20° to 150° C.,preferably 50° to 120° C. In the case where an amine is reacted, thereaction is conducted at a temperature usually of up to 80° C.,preferably 0° to 70° C. An amine or tin catalyst generally used forusual urethane forming reactions may be used to accelerate the reaction.When a solvent is used, especially preferable to use is one which issoluble in water and generally not higher than water in boiling point.

The primary and secondary monoamines (e) are useful also as neutralizingagents for the resin (A).

Component (C): Crosslinking Agent

This agent crosslinks the components (A) and/or (B) for curing. Suitablecrosslinking agents are melamine resins for coating compositions andphenol formaldehyde resins. Although these resins can be water-solubleor hydrophobic, hydrophobic resins are preferable to achieve improvedcoating work efficiency, storage stability and moisture resistance.

Examples of suitable hydrophobic melamine resins are those having asolvent dilution ratio of up to 20, preferably up to 15, and an weightaverage molecular weight of 800 to 4000, preferably I000 to 3000. Thesolvent dilution ratio is an index representing the solubility of themelamine resin in hydrophilic solvents; the lower the ratio, the higheris the hydrophobicity. The ratio is determined by placing a 5-c.c.beaker containing 2 g of melamine resin on a sheet of paper printed inNo. 5 type, and then a solvent mixture of water/methanol (35/65 inweight ratio) is added dropwise to the resin at 25° C. with stirringuntil the printed impression becomes illegible, whereon the amount(c.c.) of mixture added is measured. The ratio is expressed in the value(c.c./g) obtained by dividing the measurement by the amount of melamineresin.

The melamine resin to be used is not limited specifically insofar as theresin has the above-specified solvent dilution ratio and molecularweight. Various etherified melamine resins are usable which include, forexample, those modified with one or at least two of methyl alcohol,ethyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol,octyl alcohol, 2-ethylhexyl alcohol, benzyl alcohol and the like.Suitable for use in the present invention are those modified with analcohol having at least 4 carbon atoms, more preferably with a C₄ to C₇alcohol. Although the amount of ether groups in the melamine resin isnot limited specifically, it is suitable that the amount be up to about5 moles, preferably about 1.5 to about 3 moles, per triazine ring.Further although the kind and amount of remaining functional groups suchas amino, imino and methylol groups are not limited specifically insofaras the resin fulfils the above requirements as to the solvent dilutionratio and molecular weight, the preferred amount of imino groups(inclusive of amino groups), as well as methylol groups, is 0.2 to 2.0moles, more preferably 0.5 to 1.5 moles, per triazine ring.

It is desired that the hydrophobic crosslinking agent be admixed with awater-soluble resin before being admixed with the components (A) and(B).

The water-soluble resin is a resin (such as acrylic resin, alkyd resin,epoxy resin or the like) having introduced therein a hydrophilic groupsuch as carboxyl (--COOH), hydroxyl (--OH), methylol (--CH₂ OH), amino(--NH₂), sulfon (--SO₃ H) or polyoxyethylene bond (--CH₂ CH₂ O--_(n)).Most common such resins are those having carboxyl introduced therein,neutralized with the above-mentioned primary or secondary amine andthereby made soluble in water.

It is suitable that the water-soluble resin be admixed with thecrosslinking agent usually in a ratio of about 20 to about 100 parts byweight, preferably about 28 to about 80 parts by weight, per 100 partsby weight of the agent, calculated as solids. These two components aremixed together by a desired method. For example, the two components areuniformly mixed togehter by a disper, homomixer, ball mill, sand mill orthe like, with a coloring pigment, extender pigment, etc. kneaded withthe components when so required. Further when needed, a small amount ofhydrophilic solvent, such as alcohol or ether solvent, can be added tothe mixture in this procedure. Next, while vigorously stirring themixture, deionized water is slowly added thereto in about 0.5 to 5 timesthe combined amount by weight of the hydrophobic melamine resin and thewater-soluble resin, whereby an aqueous dispersion of crosslinking agentis obtained in which the surfaces of melamine resin particles, which aremilky white or colored, are covered with the water-soluble resin. Whenfree from a pigment, the aqueous dispersion is about 0.05 to about 0.5μin mean particle size.

A metallic pigment and/or a coloring pigment generally used for coatingcompositions can be admixed with the aqueous coating composition of theinvention. Examples of useful metallic pigments are aluminum flakes,copper bronze flakes, iron oxide resembling mica, mica flakes, ironoxide resembling mica and coated with a metal oxide, mica flakes coatedwith a metal oxide. Examples of useful coloring pigments are inorganicpigments such as titanium dioxide, iron oxide, chromium oxide, leadchromate and carbon black; and organic pigments such as PhthalocyanineBlue, Phthalocyanine Green, carbazole violet, anthrapyrimidine yellow,flavanthrone yellow, isoindoline yellow, Indanthrone Blue anequinacridone violet.

With respect to the proportions of the components (A) to (C) of theaqueous coating composition of the invention, the ratio of the component(A)/the component (C) by weight is preferably in the range of 90/10 to60/40. The ratio of the components (A)+(C)/component (B) by weight ispreferably 95/5 to 60/40, more preferably 90/10 to 70/30, when thecoating composition is to be applied to metal substrates, or ispreferably 90/10 to 20/80, more preferably 80/20 to 30/70, forapplication to plastics.

The aqueous coating composition of the present invention is applied bythe following coating method.

Suitable substrates are the external plates of motor vehicles,especially those of passenger vehicles, buses, trucks, motor cycles,etc., whereas there is no reason to limit the application only to theseuses. These substrates may be made of metals or plastics. Metalsubstrates can be precoated with a primer or intermediate coatingcomposition.

It is desired to adjust the present aqueous coating composition to asolids content of about 10 to about 40 wt. % and to a viscosity of about800 to about 5000 cps/6 r.p.m. (B-type viscometer) in the usual mannerby adding to the composition deionized water, along with a thickener,defoaming agent, etc. as required.

The aqueous coating composition of the present invention can be usedvery suitably as a base coat composition by the two-coat one-bakemethod. More specifically, this method is practiced by applying thepresent composition to metal substrates to a thickness of about 10 toabout 50μ when dried as by air spraying, airless spraying orelectrostatic coating, drying the coating with air or hot air to avolatile content of up to 25 wt. %, preferably up to about 15 wt. %, andsubsequently applying a clear coating composition to the coating to athickness of about 15 to about 70μ when dried as by electrostatic spraycoating, followed by usual setting. The combined coating thus formed isthen heated at about 120° to about 160° C. for about 15 to about 60minutes to obtain a cured coating. In this way, the substrate can becoated with a very high work efficiency easily to form an esteticcoating.

Further plastics substrates can be coated in the same manner as themetal substrate described above after having being degreased and coatedwith a primer or the like when so required. The base and clear coatingsare cured at a temperature below the thermal deformation temperature ofthe plastics.

Although it is suitable to use the present aqueous coating compositionas a base coat composition by the two-coat one-bake method as statedabove, the present composition is also usable singly for coating.

The clear coat composition to be applied to the coating of the presentaqueous coating composition can be one already known. Examples of suchcompositions are coating compositions of the organic solvent dilutiontype comprising an amino resin-alkyd resin type, acrylic resin type,amino resin-acrylic resin type, amino resin oil-free alkyd resin type,silicone polyester resin type, fluorocarbon resin type, urethane resintype or the like. Especially preferable as clear coating compositionsfrom the viewpoint of environmental problems and savings in resourcesare those of the high-solid type having a high solids content and usablewith a reduced amount of organic solvent. Powder coating compositionsare also usable.

Especially preferable clear coating compositions of the high-solid typeare those of the organic solvent thermosetting type consisting primarilyof an acrylic resin, hexaalkoxymethylmelamine and acid catalyst.

The acrylic resin for use in these clear coating compositions of thehigh-solid type comprises a monoester of acrylic acid or methacrylicacid and a monohydric alcohol having 1 to 20 carbon atoms as maincomponents, and, when required, a carboxyl-containing vinyl monomer orother vinyl monomer or the like, and is prepared by polymerizing thesecomponents. Preferably, the acrylic resin is about 1,000 to about100,000 in number average molecular weight, 50 to 200 in hydroxyl valueand 0 to 50 in acid value.

Furthermore, the acrylic resin can be modified with a reaction productof e-caprolactone and a hydroxyl-containing vinyl monomer or is usablein combination with a ring-opened polyester of ε-caprolactone.

The hexaalkoxymethylmelamine is a crosslinking agent for the acrylicresin and is obtained by fully etherifying hexamethylolmelamine with amonohydric alcohol having 1 to 4 carbon atoms. It is required that thesix methylol groups on the triazine ring be all alkyletherified. Thealkyl groups on the ring may be the same or different. Examples ofsuitable alkyl groups are methyl, ethyl, n-butyl, isobutyl and the like.

The acid catalyst serves to accelerate the crosslinking reaction betweenthe acrylic resin and the hexaalkoxymethylmelamine, and is, for example,dodecylbenzenesulfonic acid, p-toluenesulfonic acid,dinonylnaphthalenedisulfonic acid or the like. Also usable is such anacid as blocked with an amine.

The organic solvent serves to dissolve or disperse the above components,and is at least one of those used for coating compositions and selected,for example, from among hydrocarbon, alcohol, ester, ketone, ether andlike solvents.

The proportions by weight of the acrylic resin and thehexaalkoxymethylmelamine for forming the high-solid clear coatingcomposition if 45 to 85% of the former and 55 to 15% of the latter basedon the combined amount of the two components. To be suitable, the amountof acid catalyst is 0.1 to 5 parts by weight per 100 parts by weight ofthe two components combined. When to be applied, the clear coatingcomposition has a solids concentration which is preferably in the rangeof 40 to 65 wt. % although variable as desired depending on thesubstrate.

When required, the above-mentioned coloring pigment can be admixed withthe components of the clear coating composition insofar as thetransparency of the composition is not impaired. Also suitably usableare additives such as ultraviolet absorber, light stabilizer and surfaceleveling agent.

The present invention will be described in greater detail with referenceto the following preparation examples, examples and comparativeexamples, in which parts and percentages are all by weight.

PREPARATION EXAMPLE 1

Emulsions (A-1) to (A-6) of the core/shell type were prepared by thesteps (I) to (III) described below using the components listed in Table1.

Step (I): Deionized water (120 parts) was placed into a flask and heatedat 80° to 85° C. Two parts of the first pre-emulsion listed in Table 1was added to the water with stirring, the mixture was aged for 20minutes, and the remaining portion of the pre-emulsion was then addeddropwise to the mixture at the same temperature at a constant rate overa period of 3 hours to obtain an aqueous dispersion ofthree-dimensionally crosslinked core portion.

Step (II): After the completion of dropwise addition of the firstpre-emulsion, a silane monomer and allyl methacrylate were rapidly addeddropwise to the dispersion, and the mixture was maintained at 80° to 85°C. for 1 hour to react the silane monomer and the allyl methacrylate tothe surface of the core portion.

Step (III): Deionized water (50 parts) was admixed with the reactionmixture, and the second pre-emulsion listed in Table 1 was addeddropwise to the resulting mixture at 80° to 85° C. at a constant rateover a period of 1 hour.

The mixture was maintained at the same temperature for 1 hour and thenrapidly cooled to room temperature, and deiozed water was added to asolids content of 30%. The resulting polymer in the form of fineparticles was insoluble in organic solvent. Deionized water wasthereafter added to the mixture, and the mixture was neutralized withdiethanolamine and adjusted to a solids content of 20%, affording anemulsion (A-1) of the core/shell type. The same steps as above wasrepeated to obtain like emulsions (A-2) to (A-6).

The terms marked with *1 to *4 in Table 1 means the following.

(*1) Pre-emulsion: A uniform emulsion prepared by treating the mixtureof listed components by a high-speed stirrer.

(*2) The weight ratio based on the polymerizable monomers contained ineach pre-emulsion.

(*3) Diethanolamine was used for neutralization.

(*4) Determined by laser correlation spectroscopy.

                                      TABLE 1                                     __________________________________________________________________________                       Emulsion                                                   Item               A-1 A-2 A-3 A-4 A-5 A-6                                    __________________________________________________________________________    Step (I) First pre-emulsion (*1)                                              γ-Methacryloxypropyltrimethoxysilane                                                       2   4       2   4                                          Vinyltrimethoxysilane      2           2                                      Styrene            20  20  20  20  20  20                                     Methyl methacrylate                                                                              20  23  25  23  20  23                                     Butyl acrylate     43  43  40  40  43  45                                     Hydroxypropyl methacrylate                                                                       15  10  13  15  13  10                                     Polyoxyethylene nonylphenyl ether                                                                1.5 1.5 1.5 1.5 1.5 1.5                                    sulfate ammonium salt                                                         Ammonium persulfate                                                                              0.4 0.4 0.4 0.4 0.4 0.4                                    Deionized water    45  45  45  45  45  45                                     Step (II)                                                                     γ-Methacryloxypropyltrimethoxysilane                                                       2   4       2   2   2                                      Vinyltrimethoxysilane      12                                                 Allyl methacrylate 2   2   3   2   2   2                                      Step (III) Second pre-emulsion (*1)                                           Acrylic acid       16      18  16  16                                         Methacrylic acid       16              20                                     Styrene            12  12  12  12  12  12                                     Methyl methacrylate                                                                              20  18  18  24  22  16                                     Butyl acrylate     44  44  40  40  40  40                                     Hydroxyethyl acrylate                                                                            8   10  12  8   10  12                                     Polyoxyethylene nonylphenyl ether                                                                0.3 0.3 0.3 0.3 0.3 0.3                                    sulfate ammonium salt                                                         Ammonium persulfate                                                                              0.2 0.2 0.2 0.2 0.2 0.2                                    Deionized water    12  12  12  12  12  12                                     First pre-emulsion/second pre-emulsion                                                           80/20                                                                             80/20                                                                             80/20                                                                             70/30                                                                             80/20                                                                             90/10                                  (monomer wt. ratio) (*2)                                                      Before neutralization                                                         Particle size (nm) (*4)                                                                          121 131 130 110 135 103                                    pH                 2.3 2.4 2.2 2.6 2.8 2.5                                    After neutralization (*3)                                                     Particle size (nm) (*4)                                                                          124 132 135 118 138 110                                    pH                 7.5 7.5 7.5 7.5 7.5 7.5                                    __________________________________________________________________________

PREPARATION EXAMPLE 2

Into a reactor were placed 140 parts of deionized water, 2.5 parts of30% Newcol 707SF and 1 part of monomer mixture (1) given below, whichwere mixed together by stirring in a nitrogen stream, and 3 parts of 3%ammonium persulate was added to the mixture at 60° C. The mixture wasthen heated to a temperature of 80° C., and a monomer emulsioncomprising 79 parts of the monomer mixture (1), 2.5 parts of 30% Newcol707SF, 4 parts of 3% ammonium persulfate and 42 parts of deionized waterwas thereafter placed into the reactor over a period of 4 hours using ametering pump. After the completion of the addition, the mixture wasaged for 1 hour.

A monomer mixture (2) given below was further placed into the reactor at80° C. over a period of 1.5 hours, followed by aging for 1 hour, and themixture was filtered with 200-mesh nylong cloth at 30° C. Deionizedwater was further added to the filtrate, and the pH of the mixture wasadjusted to 7.5 with diethanol amine to obtain a 20% aqueous dispersion(A-7) of acrylic resin, 46° C. in Tg.

    ______________________________________                                        Monomer mixture (1)                                                           Methyl methacrylate    55    parts                                            Styrene                10    parts                                            n-Butyl acrylate       9     parts                                            2-Hydroxyethyl acrylate                                                                              5     parts                                            Methacrylic acid       1     part                                             Monomer mixture (2)                                                           Methyl methacrylate    5     parts                                            n-Butyl acrylate       7     parts                                            2-Ethylhexyl acrylate  5     parts                                            Methacrylic acid       3     parts                                            30% Newcol 707SF       0.5   part                                             3% Aqueous solution of 4     parts                                            ammonium persulfate                                                           Deionized water        30    parts                                            ______________________________________                                    

PREPARATION EXAMPLE 3

Into a reactor were added 60 parts of butyl cellosolve and 15 parts ofisobutyl alcohol, and the mixture was heated to 115° C. in a nitrogenstream. To the mixture heated to 115° C. was then added over a period of3 hours a mixture of 26 parts of n-butyl acrylate, 47 parts of methylmethacrylate, 10 parts of styrene, 10 parts of 2-hydroxyethylmethacrylate , 6 parts of acrylic acid and 1 part ofazoisobutyronitrile. The resulting mixture was thereafter aged at 115°C. for 30 minutes, and a mixture of 1 part of azobisisobutyronitrile and115 parts of butyl cellosolve was added to the aged mixture over aperiod of 1 hour, followed by aging for 30 minutes and thereafter byfiltration with 200-mesh nylon cloth at 50° C.

The reaction product obtained was 48 in acid value, Z₄ in viscosity(Gardner bubble viscometer), 55% in nonvolatile content and 45 C in Tg.The product was neutralized with an equivalent quantity ofdiethanolamine, followed by addition of deionized water, to obtain a 50%aqueous solution (A-8) of acrylic resin.

PREPARATION EXAMPLE 4

Into a pressure reactor were added 88.2 parts of polycaprolactone diol(2000 in molecular weight), 88.2 parts of poly-3-methylpentane adipatediol (2000 in molecular weight), 6.2 parts of 1,4-butanediol, 4.8 partsof trimethylolpropane, 10.0 parts of dimethylolpropionic acid, 78.6parts of IPDI and 120 parts of acetone, which were then reacted at 85°C. under an increased pressure for 5 hours. The reaction mixture wasthereafter reacted with 6.0 parts of 1,4-butanediol and 150 parts ofacetone for 3 hours at the same temperature. Subsequently, the resultingreaction mixture was cooled to 40° C., neutralized with 8.8 parts ofdiethanolamine and then admixed with 700 parts of ion-exchanged water.The acetone was distilled off from the mixture in a vacuum at atemperature of up to 70° C., giving 949 parts of a urethane resinemulsion (B-1) 31.6% in solids content, 250 cps/25° C. in viscosity and9.4 in pH.

PREPARATION EXAMPLE 5

Into a pressure reactor were placed 69.6 parts of polycarbonate diol(2000 in molecular weight), 69.6 parts of poly-3-methylpentane adipatediol (2000 in molecular weight), 2.6 parts of trimethylolpropane, 37.7parts of ethylene oxide 2-mole adduct (330 in molecular weight) ofbisphenol A, 8.6 parts of dimethylolpropionic acid, 92.0 parts of IPDIand 120 parts of acetone, which were then reacted at 85° C. for 5 hoursto obtain a prepolymer solution having a terminal NCO content of 2.68%.The solution was subsequently cooled to 35° C., and a mixture of 8.3parts of aminoethylethanolamine, 30 parts of isopropanol and 120 partsof acetone was reacted with the solution for 1 hour. The reactionmixture was then admixed with 700 parts of ion-exchanged watercontaining 15.0 parts of diethanolamine, followed by removal of thesolvent in the same manner as in Preparation Example 4, whereby 997parts of a urethane resin emulsion (B-2) was obtained which was 30.1% insolids content, 350 cps/25° C. in viscosity and 9.4 in pH.

PREPARATION EXAMPLE 6

7.9 parts of aminoethylethanolamine, 30 parts of isopropanol and 120parts of acetone was added to a prepolymer solution (NCO% 2.72) preparedfrom 66.1 parts of polyethylene isophthalate diol (2000 in molecularweight), 66.1 parts of neopentyl adipate diol (2000 in molecularweight), 2.5 parts of trimethylolpropane, 38.8 parts of ethylene oxide2-mole adduct of bisphenol A, 8.2 parts of dimethylolpropionic acid,102.0 parts of hydrogenated MDI and 120 parts of acetone, followed byreaction at 30° C. for 1 hour. The reaction mixture was neutralized with14.3 parts of diethanolamine and thereafter admixed with 700 parts ofion-exchanged water. The same procedure as in Preparation Example 4 wasthereafter followed to obtain 990 parts of a urethane resin emulsion(B-3), 30.3% in solids content, 150 cps/25° C. in viscosity and 9.2 inpH.

PREPARATION EXAMPLE 7

Into a stirring container were placed 41.7 parts of a hydrophobicmelamine resin, i.e., "UBAN 28SE" (product of Mitsui Toatsu ChemicalsInc., 60% in nonvolatile content, 0.4 in solvent dilution ratio), and 20parts of the acrylic resin aqueous solution (A-8) prepared inPreparation Example 3. While stirring the mixture by a disper at 1000 to1500 r.p.m., 80 parts of deionized water was slowly added to themixture, followed by stirring further for 30 minutes to obtain acrosslinking agent (C-1) in the form of an aqueous dispersion with amean particle size of 0.11μ.

PREPARATION EXAMPLE 8

Into a stirring container were placed 23 parts of aluminum paste (65% inmetal content) and 25 parts of butyl cellosolve, followed by stirringfor 1 hour to obtain an aluminum pigment concentrate (P-1).

PREPARATION EXAMPLE 9 (i) Preparation of Acrylic Resin (1)

Into a reactor was placed 40 parts of cellosolve acetate, which wasstirred with heating to 185° C., whereupon the following monomer mixturewas placed into the reactor over a period of 8 hours.

    ______________________________________                                        Styrene                 10    parts                                           Isobutyl methacrylate   80    parts                                           n-Butyl methacrylate    12    parts                                           2-Ethylhexyl methacrylate                                                                             20    parts                                           2-Hydroxyethyl methacrylate                                                                           25    parts                                           Methacrylic acid        8     parts                                           Cellosolve acetate      50    parts                                           α,α-Azobisisobutyronitrile                                                                4     parts                                           ______________________________________                                    

The reaction was continued further for 1 hour after the addition of themonomer mixture at the same temperature of 185° C., and a mixture of 10parts of cellosolve acetate and 0.6 part of α,α-azobisisobutyronitrilewas thereafter added to the reaction mixture over a period of 2 hoursand 20 minutes, followed by reaction for 2 hours and further bydistillation in a vacuum for the removal of the cellosolve acetate toadjust the resulting solution to a resin concentration of 65% and obtainan acrylic resin solution (1). The resin component of the solution (1)was 4,800 in number average molecular weight.

(ii) Preparation of Acrylic Resin Nonaqueous Dispersion

Into a reactor were placed 35 parts of Cymel 235 (product of MitsuiCyanamid, hexaalkoxy(methoxy/ butoxy mixture) methylmelamine), 103 partsof n-heptane and 0.15 part of benzoyl peroxide, which were heated to 95°C. The following monomer mixture was added dropwise to the mixture overa period of 8 hours.

    ______________________________________                                        Styrene                 15    parts                                           Acrylonitrile           9     parts                                           Methyl methacrylate     13    parts                                           Methyl acrylate         15    parts                                           n-Butyl methacrylate    1.8   parts                                           2-Hydroxyethyl methacrylate                                                                           10    parts                                           Acrylic acid            1.2   parts                                           Benzoyl peroxide        0.5   part                                            n-Butanol               5     parts                                           Shellsol 140            30    parts                                           (product of Shell Oil,                                                        hydrocarbon solvent)                                                          n-Heptane               9     parts                                           ______________________________________                                    

One hour after the addition of the monomer mixture, a mixture of 0.65part of tert-butyl peroctoate and 3.6 parts of Shellsol 140 was added tothe reaction mixture over a period of 1 hour, followed by continuedstirring at the same temperature of 95° C. for 2 hours. Thereafter 34parts of the solvent was distilled off in a vacuum to obtain an acrylicresin nonaqueous dispersion having a resin content of 60% and varnishviscosity A (Gardner bubble viscometer).

(iii) Preparation of Acrylic Resin (2)

An acrylic resin solution (2) having a number average molecular weightof 4600 and a resin solids content of 50% was prepared by reacting 30parts of styrene, 40 parts of n-butyl methacrylate, 10 parts of2-ethylhexyl acrylate, 18 parts of 2-hydroxyethyl acrylate and 2 partsof acrylic acid in an organic solvent mixture of 85 parts of xylol and15 parts of n-butanol.

(iv) Preparation of Acrylic Resin (3)

Into a reactor were placed 30 parts of Swasol-1000 (aromatic hydrocarbonsolvent manufactured by Cosmo Oil Co., Ltd.), 20 parts of xylol and 15parts of n-butanol, which were then heated to 130° C. with stirring,whereupon the following monomer mixture was added to the above mixtureover a period of 3 hours.

    ______________________________________                                        Styrene                 20    parts                                           n-Butyl methacrylate    22    parts                                           n-Butyl acrylate        20    parts                                           2-Hydroxyethyl acrylate 16    parts                                           TONE M-100.sup.1)       20    parts                                           Acrylic acid            2     parts                                           α,α-Azobisisobutyronitrile                                                                2.2   parts                                           ______________________________________                                    

After the completion of addition of the above monomer mixture, theresulting mixture was maintained at 130° C. further for 1 hour, followedby the addition of a mixture of 1 part of tert-butyl peroctoate and 10parts of xylol over a period of 1 hour and thereafter by continuedstirring at 130° C. for 2 hours. The mixture was then cooled, and xylolwas added thereto to obtain an acrylic resin solution (3) having asolids concentration of 50 wt. %. The acrylic resin was 7500 in numberaverage molecular weight (Mw) and 110 in hydroxyl value.

1) TONE M-100: Brand name of Union Carbide (U.S.) for a monomer which isan adduct of 1 mole of 2-hydroxyethyl acrylate with 2 moles ofε-caprolactone.

Clear coating compositions (T-1) to (T-3) of the high-solid type eachcomprising the components listed in Table 2 were prepared using theacrylic resins (1) to (3) and acrylic resin nonaqueous dispersionobtained by the procedures (i) to (iv) above.

                  TABLE 2                                                         ______________________________________                                                        Alkoxy-   Acid      Solids                                    Acrylic resin   melamine  Catalyst  content                                   (parts)         (parts)   (part)    (%)                                       ______________________________________                                        T-1   (1)         77    M-1 20  CAT-1 0.5                                                                             55                                          Nonaqueous  50                                                                dispersion                                                              T-2   (2)        150    M-2 25  CAT-2 0.5                                                                             55                                    T-3   (3)        140    M-3 30  CAT-3 0.5                                                                             55                                    ______________________________________                                    

The alkoxymelamines and the acid catalysts given in Table 2 are asfollows.

Alkoxymelamines

M-1: Cymel 303 (hexamethoxymethylmelamine manufactured by MitsuiCyanamide Co., Ltd.)

M-2: Cymel 235 (methoxy/butoxy fully etherified monomeric melamine bymanufactured by Mitsui Cyanamide Co., Ltd.)

Acid catalysts

CAT-1: Catalyst 6000 (dodecylbenzenesulfonic acid manufactured by MitsuiToatsu Chemicals Inc.)

CAT-2: Nacure 5225 (amine block of dodecylbenzene-sulfonic acidmanufactured by King Industries (U.S.))

CAT-3: Nacure 2500 (amine block of p-toluenesulfonic acid manufacturedby King Industries (U.S.))

EXAMPLES 1-9 AND COMPARATIVE EXAMPLES 1-3

Aqueous coating compositions were prepared each by mixing together someof the components obtained in Preparation Examples 1 to 8 in theproportions listed in Table 3 and adding "Acrysol ASE-60" (thickenermanufactured by Rohm & Haas Co.) and diethanolamine to the mixture toadjust the mixture to an apparent viscosity of 3000 cps/6 r.p.m. (B-typeviscometer) and a pH of 7.80.

The amounts listed in Table 3 are each the amount of solids.

Although the component (C-1) includes the component (A-8), the amount ofcomponent (C) given in the table is the amount of hydrophobic melamineresin only, and the amount of component (A-8) is included in that of thecomponent (A). Accordingly, the amount in the column of the component(A) is the sum of the amount of (A-8) included in the component (C-1)and the amount of the particular component (A) mentioned in the column.Thus, the amount of component 9A) is the sum minus the amount ofcomponent (A-8).

The components (B-1)', (B-2)' and (B-3)' given for Comparative Examplesare triethylamine as used in place of, and in the same equivalent amountas, the primary or secondary amine used for the respective components(B-1), (B-2) and (B-3).

                                      TABLE 3                                     __________________________________________________________________________    Component (A)  Component (B)                                                                          Component (C)                                                                          Component                                              Amount   Amount   Amount                                                                             (P-1)                                        Kind      (parts)                                                                            Kind                                                                              (parts)                                                                            Kind                                                                              (parts)                                                                            (parts)                                      __________________________________________________________________________    Example                                                                       1     (A-1)                                                                             80   (B-1)                                                                             17   (C-1)                                                                             20   15                                           2     (A-2)                                                                             80   (B-1)                                                                             33   (C-1)                                                                             20   15                                           3     (A-3)                                                                             75   (B-2)                                                                             17   (C-1)                                                                             25   15                                           4     (A-4)                                                                             75   (B-2)                                                                             33   (C-1)                                                                             25   15                                           5     (A-5)                                                                             70   (B-3)                                                                             17   (C-1)                                                                             30   15                                           6     (A-6)                                                                             70   (B-3)                                                                             33   (C-1)                                                                             30   15                                           7     (A-7)                                                                             60   (B-1)                                                                             25   (C-1)                                                                             20   15                                                 (A-8)                                                                             20                                                                  8     (A-7)                                                                             55   (B-2)                                                                             25   (C-1)                                                                             25   15                                                 (A-8)                                                                             20                                                                  9     (A-7)                                                                             55   (B-3)                                                                             25   (C-1)                                                                             30   15                                                 (A-8)                                                                             15                                                                  Comp. Ex.                                                                     1     (A-1)                                                                             80    (B-1)'                                                                           17   (C-1)                                                                             20   15                                           2     (A-3)                                                                             75    (B-1)'                                                                           17   (C-1)                                                                             25   15                                           3     (A-5)                                                                             70    (B-3)'                                                                           17   (C-1)                                                                             30   15                                           __________________________________________________________________________

COATING METHOD

Steel panel substrates were coated by the two-coat one-bake method asdescribed below using the aqueous coating compositions (for base coats)prepared in Examples 1 to 9 and Comparative Examples 1 to 3 and theclear coating composition of the high-solid type obtained in PreparationExample 9.

Each of the substrates was prepared by surface-treating a steel panelwith "Bonderite #3030" (zinc phosphate treating agent, product of NihonParkerizing Co., Ltd.), electrophoretically coating the panel with aprimer, i.e., "Elecron No. 9200" (epoxy resin cationic electrophoreticcoating composition manufactured by Kansai Paint Co., ltd.) and applying"Amilac n-2 sealer" (aminopolyeaster resin intermediate coatingcomposition manufactured by Kansai Paint Co., Ltd.) to the coating.

The base coating composition was applied twice to the substrate in anenvironment having a temperature of 25° C. and a relative humidity of65% by spray coating with a spray gun with an interval of 2 minutesprovided between the two procedures for setting.; the spray gun was usedat an air pressure of 5 kg/cm² and a composition flow rate of 350 ml/minat a distance of 35 cm from the substrate. The substrate was held in avertical position during the entire coating operation. After the secondapplication of the composition, the coated substrate was allowed tostand in the same environment for 2 minutes, then dried in air at atemperature of 80° C. for 10 minutes and cooled to room temperature. Thesubstrate was thereafter coated with the clear coating compositionobtained in Preparation Example 9 using an electrostatic gun, followedby setting for 5 minutes and baking at 120 to 140° C. for 30 minutes.The base coating composition and the clear coating composition wereapplied to thicknesses of 15 μm and 40 μm, respectively, calculated asdry coatings. In this way, all the substrates were coated by thetwo-coat one-bake method.

The base coating compositions and the coatings formed were tested forthe following properties (1) to (12). Table 4 shows the results.

(1) Sag

A cavity 10 mm in diameter, was formed in the substrate, which was thencoated and checked for the sagging of the composition. The mark ∘represents 0- to 2-mm sag, the mark Δ 2- to 4-mm sag, and the mark X 4-to 6-mm sag.

(2) Irregularities

The coating was checked for surface irregularities. The mark ∘represents almost no irregularity, the mark Δ some slightirregularities, and the mark x marked irregularities.

(3) Chipping Resistance

Determined using a Gravelometer tester ("Model JA-400," product of SugaShikenki Co., Ltd.). The coated test panel was attached vertically tothe sample holder of the tester, and 50 g of No. 7 crushed stones werepneumatically forced out at an air pressure of 4 kg/cm² by pressuremeans on the tester against the test panel perpendicular thereto. Thetest panel was thereafter washed with water and dried, the coatingportions raised by chipping were removed with an adhesive tape, and thecoating was then checked for peel flaws according to the followingcriteria.

    ______________________________________                                        Cri-     Size of scales of                                                                         Number of scales in area                                 teria    overcoat    of 3 cm square                                           ______________________________________                                        1        Overcoat was free from scaling but had some                                   flaws.                                                               2        Up to 0.5 mm                                                                              Up to 10                                                 3        0.5-1.0 mm  20-40                                                    4        1.0-2.0 mm  30-60                                                    5        1.5-3.0 mm  50-80                                                    ______________________________________                                    

Before the chipping resistance test, the test panel was immersed in dryice/methanol cooled to -25° C. for 5 minutes to 10 minutes, whereuponthe panel was withdrawn and tested immediately (within several seconds).The test panel had a temperature of -20°±5° C. during the testing.

(4) Humidity Resistance

The coated test panel was allowed to stand for 240 hours in a chambermaintained at a relative humidity of 98% and a temperature of 49°±1° C.using a humidity tester (Suga Shikenki Co., Ltd.).

Water drops were wiped off immediately after the test panel waswithdrawn from the chamber, and the overcoat was checked for blisters,shrinkage and other faults. The mark ∘ indicates that the humidity testproduced no change in the coating. The mark Δ indicates some blisters,shrinkage or other faults.

(5) Impact Resistance

Using a Du Pont impact tester, a 500-gram weight with a tip radius of1/2 inch was dropped onto the coated panel to determine the maximumdistance (cm) of fall which caused no cracking in the coating. Theresistance is expressed in terms of this distance.

(6) Weather Resistance

Determined by QUV accelerated exposure test using an acceleratedweathering tester, product of Q Panel Co., Ltd.

The coated panel was tested under the conditions of:

    ______________________________________                                        irradiation with UV rays                                                                        16 h/60° C.                                          condensation of water                                                                            9 h/50° C.                                          ______________________________________                                    

as one cycle for 3600 hours (150 cycles), and the coating was thereafterchecked. The mark ∘ indicates that the coating remained almost unchangedin gloss, the mark Δ represents a reduction in gloss but none of defectssuch as cracks and chalking, and the mark X represents a markedreduction in gloss, cracks and chalking, hence a reject.

(7) Solvent Resistance

The coating on the coated panel was forcibly rubbed reciprocatingly 20times with gauze wetted with xylol and pressed against the panel withthe finger. The result was evaluated according to the three criteria ofgood, fair and poor, respectively represented by ∘, Δ and X, dependingon the degree of melting or swelling of the coating and flaws therein.

(8) Water Resistance

The coated panel was immersed in water at 40° C. for 10 days, and thecoating was thereafter checked. No change is represented by ∘, slightblistering by Δ, and marked blistering by X.

(9) Interlayer Adhesion

Approximately in the center of the coated panel, eleven parellel cutlines were formed at a spacing of 1 mm both vertically and horizontallyin the coating to the surface of the substrate using a cutter knife toform 100 squares within an area of 1 cm². An adhesive cellophane tapewas then affixed to the cross-cut coating and subsequently peeled offquickly, and the cross-cut area was checked. No separation of thecoating is represented by ∘, slightly noticeable separation between themetallic coating and the clear coating by Δ,and marked interlayerseparation by X.

(10) Distinctness-Of-Image Gloss

Measured by an image clarity meter (product of Suga Shikenki Co., Ltd.).The figures in the table are ICM values in the range of 0 to 100%. Thegreater the numerical value, the better is the distinctness-of-imagegloss. ICM values of not lower than 80 represent excellentdistinctness-of-image gloss.

(11) Smoothness

Determined with the unaided eye.

(12) Metallic appearance

Determined with the unaided eye.

                                      TABLE 4                                     __________________________________________________________________________    Example                                   Comp. Ex.                           Properties                                                                          1   2   3   4   5   6   7   8   9   1   2   3                           __________________________________________________________________________    Clear T-1 T-2 T-3 T-1 T-2 T-3 T-1 T-2 T-3 T-1 T-2 T-3                         coating                                                                       composition                                                                   (1)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           Δ                                                                           Δ                     (2)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯               (3)    1   1   1   1   1   1   1   1   1   2   2   2                          (4)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           Δ                                                                           Δ                     (5)   35  35  35  35  35  35  35  35  35  25  25  25                          (6)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           Δ                                                                           Δ                     (7)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X   X   X                           (8)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           Δ                                                                           Δ                     (9)   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           Δ                                                                           Δ                     (10)  89  88  88  90  88  89  90  90  88  78  76  77                          (11)  Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                                                              Poor                                                                              Poor                        (12)  Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Good                                                                              Poor                                                                              Poor                                                                              Poor                        __________________________________________________________________________

We claim:
 1. A two-coat one-bake coating method comprising the stepsof:(1) applying a base coating composition to a substrate to form afirst coating; (2) without curing the first coating, applying a clearcoating composition to the first coating to form a transparent coatingthereon; and (3) thereafter heating both the first coating and thetransparent coating for curing, wherein said base coating compositionconsists essentially of a pigment and an aqueous resin composition, saidpigment being at least one selected from the group consisting ofcoloring pigments and metallic pigments, and said aqueous resincomposition consisting essentially of:(A) a water-soluble orwater-dispersible resin, (B) a urethane resin emulsion, and (C) acrosslinking agent, the component (A) being neutralized with at leastone amine selected form the group consisting of primary and secondarymonoamines, the component (B) being an aqueous dispersion of apolyurethane resin prepared by reacting (a) at least one polyisocyanateselected from the group consisting of aliphatic polyisocyanates andalicyclic polyisocyanates, (b) a high-molecular-weight polyol, (c) anα,α-dimethyolmonocarboxylic acid, and if necessary (d) at least onemember selected from the group consisting of chain lengthening agentsand polymerization terminators, and subsequently neutralizing thecarboxyl groups in the reaction product with at least one amine selectedfrom the group consisting of primary and secondary monoamines; andwherein said clear coating composition is a thermosetting high-solidclear coating composition consisting essentially of an acrylic resin, ahexaalkoxymethylmelamine, an acid catalyst and an organic solvent.
 2. Acoating method as in claim 1, wherein the resin (A) is an acrylic resin.